Fayçal Rejiba

This study focuses on characterising a strong and unexpected anomaly observed during an electromagnetic induction (EMI) survey conducted on the chalk cliff plateau at Sainte‐Marguerite‐sur‐Mer, in the Normandy region, France. The site was selected for a long‐term study of chalk cliff coastal erosion that began in 2017 and has been studied for stability risk assessment through hydrogeophysical and thermal approaches since 2022. Sainte‐Marguerite‐sur‐Mer also has significant historical relevance due to its occupation during World War II and the construction of the Atlantic Wall. A bunker from this period is located on the shore platform at the study site, and other bunkers remain visible south of the survey area, near the village. According to local reports, the 340‐ anomaly was initially suspected to overlie an old municipal landfill. Coarse geophysical imaging at 10 m spacing was initially conducted, followed by finer geophysical surveys at 1 m spacing and continuous monitoring, using, for instance, multi‐depth TDR sensors (time‐domain reflectometry) to follow near‐surface soil properties. Various methods were employed to characterise the properties of this anomaly with greater precision, including electrical resistivity tomography (ERT), EMI and ground‐penetrating radar (GPR). Recent high‐resolution geophysical conductivity maps have revealed a linear anomaly of less than 50 extending from the cliff inland over 50 m, suggesting an anthropogenic origin. The typical geometry of a buried pipe, rather than a landfill site, led our research to further investigations through simulations to estimate its diameter, conductivity and magnetic permeability. Physical properties were simulated for GPR, ERT and EMI methods. This pipe‐shaped buried network could therefore be related to the WWII military defence systems identified at the site. The conductive linear anomaly may correspond to old cable trenches carrying telephone or electrical wires that once connected the bunkers before being abandoned.

– Specific EMI instrumental drift compensation processing.– High-resolution EMI in-phase mapping highlights buried archaeological structures.– EMI quadrature component mostly delineates local geological variability.

Renovation of water and central heating pipelines is a very costly and time‐consuming process; therefore, a way to prioritize the limited resources between different parts of the systems is very important. The risk for corrosion damage can be assessed from the resistivity of the ground, because the processes facilitating the metal oxidation also affect the resistivity. However, galvanic resistivity mapping is time consuming and work‐intensive in paved areas. To determine the resistivity in the vicinity of pipes two different resistivity methods were applied: electrical resistivity tomography using galvanic coupling, and the logistically easier and rapid electrostatic measurements using capacitive coupling. The two methods were tested in a series of experiments undertaken in the province of Scania in southern Sweden with the aim to acquire a better knowledge about the electrical resistivity of the soil surrounding the heating and water distribution pipes, in order to better assess the corrosivity of the environment. From the experiments it is shown that the electrical resistivity tomography and electrostatic methods mostly give comparable results for the shallow investigated depths in focus here, where differences might be caused by different sensitivities and noise characteristics. In the case of both methods, it is shown, with the help of modelling of the different expected ground models including the pipes, that the pipes only influence the data in cases of pipes of very large diameters or those buried at a very shallow depth, even without any protective surface coating. The missing influence of the pipes on the data makes the methods very applicable for knowing the resistivity of the soil surrounding the pipes and thus evaluation of corrosion risk.

The semi‐arid Plio‐Quaternary aquifer of the Kairouan Plain is located in the central part of Tunisia. In this region, the main form of human activity is extensive agriculture. The few lithological logs in this area indicate that the aquifer is divided into two groundwater reservoirs separated by a confining clay layer. In the shallower groundwater reservoir, the electrical conductivity of the water is close to 1,200 mS/m, that is, 3 to 4 times higher than that of the deeper aquifer, which is currently used for irrigation. Recently, an increase in salinity has been observed, which is producing significant difficulties with irrigation. In order to determine the geometry of the confining clay layer and assess the potential connectivity between both reservoirs, time‐domain electromagnetic (TDEM) and vertical electrical soundings surveys were carried out. In the extremely conductive environment (electrical resistivities <10 Ωm) of the Sebkha Kelbia, the TDEM survey produced the first geophysical map of this near‐surface aquifer system.

In previous studies, the Auxiliary Differential Equation (ADE) method has been applied to 2-dimensional (2D) seismic-wave propagation modelling in viscoelastic media. This method is based on the separation of the wave propagation equations derived from the constitutive law defining the stress-strain relation. We make here a 3D extension of a finite-difference (FD) scheme to solve a system of separated equations consisting in the stress-strain rheological relation, the strain-velocity and the velocity-stress equations. The current 3D FD scheme consists in the discretization of the second order formulation of a nonlinear viscoelastic wave equation with a time actualization of the velocity and displacement fields. Compared to the usual memory variable formalism, the ADE method allows flexible implementation of complex expressions of the desired rheological model such as attenuation/viscoelastic models or even nonlinear behaviours, with physical parameters that can be provided from dispersion analysis. The method can also be associated with optimized perfectly matched layers-based boundary conditions that can be seen as additional attenuation (viscoelastic) terms. We present the results obtained for a nonlinear viscoelastic model made of a Zener viscoelastic body associated with a nonlinear quadratic strain term. Such nonlinearity is relevant to define unconsolidated granular model behaviour. Thanks to a simple model, but without loss of generality, we demonstrate the accuracy of the proposed numerical approach.

The La Bassée floodplain area is a large groundwater reservoir controlling most of the water exchanged between local aquifers and hydrographic networks within the Seine River basin (France). Preferential flows depend essentially on the heterogeneity of alluvial plain infilling, whose characteristics are strongly influenced by the presence of mud plugs (paleomeander clayey infilling). These mud plugs strongly contrast with the coarse sand material that composes most of the alluvial plain, and can create permeability barriers to groundwater flows. A detailed knowledge of the global and internal geometry of such paleomeanders can thus lead to a comprehensive understanding of the long-term hydrogeological processes of the alluvial plain. A geophysical survey based on the use of electromagnetic induction was performed on a wide paleomeander, situated close to the city of Nogent-sur-Seine in France. In the present study we assess the advantages of combining several spatial offsets, together with both vertical and horizontal dipole orientations (six apparent conductivities), thereby mapping not only the spatial distribution of the paleomeander derived from lidar data but also its vertical extent and internal variability.

Core Ideas: OZCAR is a network of sites studying the critical zone. OZCAR covers various disciplines. OZCAR will help disciplines to work together for a better representation and modeling of the critical zone. The French critical zone initiative, called OZCAR (Observatoires de la Zone Critique–Application et Recherche or Critical Zone Observatories–Application and Research) is a National Research Infrastructure (RI). OZCAR‐RI is a network of instrumented sites, bringing together 21 pre‐existing research observatories monitoring different compartments of the zone situated between “the rock and the sky,” the Earth's skin or critical zone (CZ), over the long term. These observatories are regionally based and have specific initial scientific questions, monitoring strategies, databases, and modeling activities. The diversity of OZCAR‐RI observatories and sites is well representative of the heterogeneity of the CZ and of the scientific communities studying it. Despite this diversity, all OZCAR‐RI sites share a main overarching mandate, which is to monitor, understand, and predict (“earthcast”) the fluxes of water and matter of the Earth's near surface and how they will change in response to the “new climatic regime.” The vision for OZCAR strategic development aims at designing an open infrastructure, building a national CZ community able to share a systemic representation of the CZ, and educating a new generation of scientists more apt to tackle the wicked problem of the Anthropocene. OZCAR articulates around: (i) a set of common scientific questions and cross‐cutting scientific activities using the wealth of OZCAR‐RI observatories, (ii) an ambitious instrumental development program, and (iii) a better interaction between data and models to integrate the different time and spatial scales. Internationally, OZCAR‐RI aims at strengthening the CZ community by providing a model of organization for pre‐existing observatories and by offering CZ instrumented sites. OZCAR is one of two French mirrors of the European Strategy Forum on Research Infrastructure (eLTER‐ESFRI) project.